I finally have those pesky CSS issues solved but now, one last issue. Chapter Headings. Body size set at 1em. Chapter Headings set at 1.226 em. Title set at 1.666em.

Text and Title look great when running epub through KindlePreviewer and they hold their size well. But Chapter Headings are huge, even bigger than the Title (which looks like it's 1.666em as in the html... 18pts). I checked the epub and of course everything is fine there.

Any ideas? Should I just set the Chapter Headings as Body Text size (1em) and maybe bold and/or italic?

It is common to not attempt to set sizes for HTML elements and let the reader device use it's defaults. I would suggest doing that since setting the heights might be problematic depending on which device the book is displayed. Having said that I have no idea why the chapter headings are so big without seeing the CSS and HTML you used.

MOBI (standard old MOBI not KF8) doesn't do CSS, so everything in your source & CSS is being converted/combined into something resembling HTML 3.2 by kindlegen/previewer. I imagine the conversion algorithm is doing its best to scale your font-sizes appropriately, but you must keep in mind that font size in the MOBI markup language is not determined by "em"—and even the stuff that does recognize "em" (width, height attributes), doesn't recognize fractional ems. Stuff gets rounded. Font size in MOBI markup is manipulated by the <font> tag. So everything in your CSS related to font size must be converted to something like <font size="6"> or <font size="+1">

So given varying elements (p, h1, h2, h3, etc...) being styled with fractional ems via CSS it's not inconceivable that a font size could actually end up being much larger than you intended when run through the conversion process.

Use Mobi_Unpack or calibre's debug output to see what's actually happening inside the MOBI markup language and adjust accordingly. Simpler is almost exclusively better when dealing with the MOBI format. Meaning if you need different sized headings ... then keep it simple: just use stock heading tags (h1, h2, h3) without the CSS font-size property and h3 will always be smaller than h2 in the resulting MOBI.

There's ePub... and then there's ePub formatted to be the source used to build a comparable MOBI. The trick is learning how to combine the two so that either format is only a tweak or two to a single source-tree away.

You indicated you were using ems to size your chapter headers. Pts are rarely a good choice for font size.

What does your "Title" CSS look like (that is producing what you expected) by comparison?

EDIT: also, in addition to seeing the both the "Title" and the "Chapter" CSS, it would be helpful to see snippets of the html where the CSS is being applied (for both the Title and a Chapter heading). Making sure there are not two separate CSS classes being applied to one html element is the first order of business when trying to figure out why the Previewer/kindlegen isn't producing the styling you expect it to.

DiapDealer: if CSS is converted to tags for the e-ink Kindles, is a KF7 file therefore bigger than a KF8 file for the Fire and its friends?

Looking at recent purchases of mine from Amazon, yes... the file size of the stand-alone MOBI file is consistently larger than its stand-alone KF8 counterpart. Not much of a sampling to work with, but it seems logical to me.

Remember though: being "eInk" isn't the determining factor in what supports KF8 (or what gets converted to old-style, html tag attributes). The latest generation eInk Kindles support KF8 now as well. CSS is converted to tag attributes for MOBI, period. Regardless of the device reading it.

Another quick question... I see that you all use the opf file (booktitle.opf) in KindlePreviewer to make the mobi... I've tried to do that, and my KindlePreviewer balks and won't convert. The epub file (Booktitle.epub) converts fine though. What am I doing wrong?

Another quick question... I see that you all use the opf file (booktitle.opf) in KindlePreviewer to make the mobi... I've tried to do that, and my KindlePreviewer balks and won't convert. The epub file (Booktitle.epub) converts fine though. What am I doing wrong?

They do the same thing, Jon. Dragging an ePub/OPF/XHTML file onto KindlePreviewer works just the same as as feeding them to kindlegen (because kindlegen is embedded in KindlePreviewer). The only thing you can't do when converting using KindlePreviewer is choose your compression level.

@AThirstyMind: are you using a recent version of KindlePreviewer? I don't think it worked with OPF files until v2.4. Also, you must leave the OPF file where it is in relation to the rest of the ebook's files, so the Previewer can find them (which of course implies that the hrefs in the OPF file accurately point to the rest of the files). And also, the OPF file can't already be zipped up in an ePub archive. If you have an ePub—use the ePub.

Yes, absolutely. The title of this thread says "converting ePub to Mobi". If you are indeed converting an ePub file to Mobi, you don't need to worry about the OPF - just use the ePub file as input. The only time you need to worry about the OPF is when (as I do) you're creating the Mobi from "scratch", rather than converting from an already existing ePub.